1. Field of the Invention
This invention relates generally to fuel cells and, more particularly, to maintaining a compressed state of the fuel cell.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity. A variety of materials may be suited for use as a fuel depending upon nature of the fuel cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is complex, and requires expensive components, which occupy comparatively significant volume, the use of reformer based systems is presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger scale applications. In fuel cells of interest here, a carbonaceous fuel, in either liquid or vapor form, including an aqueous solutions (typically aqueous methanol) is delivered to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically conductive, but electronically non-conductive membrane (PCM). Typically, a catalyst, which enables direct oxidation of the fuel on the anode aspect of the PCM, is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen in the fuel solution involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons travel through an external circuit, which includes the load, thus providing electrical power from the fuel cell, and are united with the protons and oxygen molecules in the cathodic reaction.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised predominantly of methanol or methanol and water (in either liquid or vapor form) is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed at an acceptable rate (more specifically, slow oxidation of the fuel mixture will limit the cathodic generation of water, and vice versa).
Direct methanol fuel cells are being developed towards commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell and the other components should be fabricated using materials and processes that are not only compatible with appropriate form factors, but which are also cost effective. Furthermore, the manufacturing process associated with a given system should not be prohibitive in terms of associated labor or manufacturing cost or difficulty.
Typical DMFC systems include a fuel source, fluid and effluent management and air management systems, and a direct oxidation fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing. A typical MEA includes a centrally disposed, protonically conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is NAFION® a registered trademark of E.I. DuPont de Nemours and Company, a cation exchange membrane comprised of polyperfluorosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles, though other catalyst combinations may also be used. On each face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the fuel mixture across the anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to achieve a sufficient supply and even distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM. Notably, a passive system in which no external water collection is performed, and instead, water is pushed back from the cathode side to the anode side of a fuel cell is described in commonly owned U.S. patent application Ser. No. 10/413,983 for DIRECT OXIDATION FUEL CELL OPERATING WITH DIRECT FEED OF CONCENTRATED FUEL UNDER PASSIVE WATER MANAGEMENT, of Ren et al., filed on Apr. 15, 2003, which is presently incorporated by reference herein.
As noted, the MEA is formed of a centrally disposed PCM that is sandwiched between two catalyst layers. The catalyst layers of the MEA in some architectures can be arranged such that cathode diffusion layer is adjacent the cathodic catalyst layer to allow oxygen to be transported to the cathode, and liquid effluent managed effectively. An anode 1 diffusion layer is adjacent the anodic catalyst layer that allow fuel to be transported to the anode, and to allow carbon dioxide to travel away from the anode. Gaskets are often used to maintain the catalytic layers and the diffusion layers in place. Generally, the entire MEA is placed into a frame structure that both compresses the MEA and provides an electron path. Although this can provide some dimensional stability, the greater the compression that is required, the more physical components (i.e., screws, etc.) must be employed to assure adequate pressure. Those skilled in the art will recognize that sealing and application of significant pressure can be accomplished in various ways, but these aspects conventionally involve relatively large fastening components, such as screws, nuts and the like. Such components themselves can be expensive because they are specially machined. Furthermore, the assembly of devices that include these fasteners is a time consuming manual process that can also lead to inconsistency in results, as compared to automated commercial volume manufacturing methods. Moreover, the additional parts can add weight, volume and cost to the fuel cell, which if used as a power source for hand-held electronic devices, should be of the smallest form factor possible.
As noted, it is also common to place gasketing around the exterior portions of the fuel cell to resist leaks of the fuel substance or water that is produced at the cathode out of the fuel cell. The gasketing can also be used to retain moisture in the fuel cell, as the NAFION® membrane operates ideally when sufficiently hydrated. The gasketing that is incorporated to prevent leakages is typically a deformable plastic material that is stretched and placed around the outer current conductor plates and usually hand-assembled around the lateral portions of the fuel cell.
In accordance with commonly owned, co-pending U.S. patent application Ser. No. 10/449,271, filed on May 30, 2003, by Hirsch, et al., for a FUEL EFFICIENT MEMBRANE ELECTRODE ASSEMBLY, which is incorporated by reference herein, a direct oxidation fuel cell is described in which the catalyst layers and diffusion layers can be extended beyond and overlap the gasketing to form an even greater seal than the gasketing would alone. In addition, the catalyzed portions of the membrane are extended into the area of the gasket to substantially resist the flow of fuel substance through any paths created at the edges of the diffusion layer between the diffusion layer and the gasket. Extending the catalytic layer into the area of the gasket allows methanol or other fuel substances to be oxidized on the catalyst prior to its leaking out of or around the diffusion layer. These aides prevent undesired leakages; however, such gaskets must be manually placed in the proper location during manufacture. Additionally, the fuel cell still requires the use of screws, bolts and other fasteners to maintain the fuel cell components in place, and to maintain the proper compression required for electrical contact and leakage prevention.
More specifically, fuel cells contain a number of components. These components can include one or more of the following: a fixture or base compression plate, a gasket, an anode current collector plate, a second gasket, a membrane electrode assembly, and yet another gasket, then the cathode current collector, a further gasket and a cathode compression plate. Depending on the design of the current collectors, components be used on either the anode aspect of the MEA or the cathode aspect of the MEA or both. To create and maintain compression through all of the layers, multiple fasteners (typically, four to eight screws and nuts) are often used to create and maintain the compression through the MEA. Typical fuel cell assembly techniques involve layering the components of the fuel cell by hand and then compressing the components together by tightening the screws to achieve a desired compression. As this is accomplished by hand, such a manufacturing technique results in variations in compression from build to build, in addition to consuming significant assembly time per cell. In addition to DMFCs, other types of fuel cells, such as hydrogen-gas fueled fuel cells, conventionally require these manufacturing techniques, and have the same disadvantages.
Some of the disadvantages of these techniques are addressed in commonly-owned U.S. patent application Ser. No. 10/650,424, filed on Aug. 28, 2003, by Fannon et al., for a METHOD OF MANUFACTURING A FUEL CELL ARRAY AND RELATED ARRAY, which is incorporated by reference herein. That patent describes an injection molding process for a fuel cell array assembly in which a precompression is introduced into the assembly by applying a predetermined surface pressure with flat mold plates acting as compression plates. This precompression is applied in order to reduce the contact resistance of the current collectors of the fuel cell and to achieve a desired hydration characteristic in the MEA. In accordance with that technique, while the precompression is being applied by the compression plates, plastic is injected around the perimeter of the current collectors, and is allowed to cure. The assembly becomes an integrated structure and surface pressure is released. Since the current collector is held by the plastic frame at its perimeter, in such designs, the compression on the current collectors eventually relaxes and the current collectors can take on a 3-dimensional convex shape, with a maximum deflection occurring at the center. As a consequence, a part or all of the applied precompression at the center region is relaxed, which results in increased contact resistance between the current collector and the MEA. A small additional relaxation occurs at the boundaries caused by the stretching of the plastic frame. Maximum deflection is proportional to the third power of the thickness of the current collector. Thus, to avoid some of these disadvantages, the current collectors are made with a comparatively large thickness. These thick current collectors, however, are manufactured by an expensive process at a high cost.
In addition, when the MEA itself begins to relax, an undesired property is its creep characteristic. In a constant strain system, like the fuel cell assembly where the anode and cathode current collectors are structurally connected together at a fixed distance, creep is defined as a drop in load or pressure with time. As the MEA creeps over time, a part or all of the applied precompression is relaxed, which results in increased contact resistance of the fuel cell and less than desirable hydration characteristics of the MEA. These characteristics both reduce the performance of the fuel cell. In addition, leakage of fuel cell working liquid from the anode and cathode can occur where the MEA meets the plastic frame in such systems.
There remains a need, therefore, for a design which addresses the problem of MEA creep such that the compression on the MEA is maintained. There is a further need for a design in which the central portion of the current collector is maintained in its compressed state so that a thinner material can be used for the current collectors.
It is thus an object of the invention to provide a cost effective, highly efficient process for manufacturing a fuel cell or fuel cell array, which allows for the MEA to maintain compression and which allows for the use of thinner current collectors. It is a further object of the invention to provide a fuel cell that does not use heavy screws, bolts and other weighty metal fasteners.